Since the introduction of oseltamivir (Tamiflu(r)) to the market, oseltamivir-resistant influenza A/H1N1 and A/H3N2 viruses have occasionally been isolated from humans, usually from persons treated with the drug [41, 43, 66-69]. Previous in vitro and in vivo data suggested that drug-resistant viruses were less fit than sensitive strains, due to compromised enzymatic activity of the mutated neuraminidase (NA) [18, 48-50, 70, 71]. However, the sudden, exponential increase in the prevalence of oseltamivir resistance among seasonal A/H1N1 viruses - less than 1% resistant in 2007, 11% resistant in 2008, and near 100% resistant in 2009 [51, 52] - cannot be explained solely by selective pressure of oseltamivir use. These oseltamivir-resistant A/H1N1 viruses may have evolved a fitness advantage over sensitive strains, apart from drug resistance; specifically, resistant viruses may transmit more efficiently among mammalian hosts. Preliminary data from our laboratory supports this hypothesis. For seasonal A/H1N1 virus-es, the old paradigm - that, in acquiring oseltamivir resistance, viruses become less fit - may no longer hold true. The differential transmissibility of seasonal A/H1N1 viruses provides a timely and relevant case study to explore how drug-resistant influenza viruses evolve to overcome impaired mammalian transmissibility. The use of oseltamivir has dramatically increased during the current swine-origin A/H1N1 pandemic [72]; this has in turn increased the chances of selecting resistant viruses in novel A/H1N1-infected, oseltamivir-treated individuals. Indeed, in the seven months since the swine-origin A/H1N1 strain was first recovered from humans, more than 25 oseltamivir-resistant isolates have been detected worldwide, mainly (but not entirely) from oseltamivir-prophylaxed or -treated persons [54-56]. To date, all of these isolates have carried the same oseltamivir resistance mutation - a histidine-to-tyrosine change at residue 275 of the viral NA (NA-H275Y) - that is found in >99% of seasonal A/H1N1 viruses. Furthermore, seasonal A/H1N1 and A/H3N2 viruses have continued to co-circulate at low levels with swine-origin A/H1N1 viruses worldwide [56]; genetic reassortment among these strains could theoretically yield a novel human virus, with an oseltamivir-resistant N1 NA in combination with other seasonal and swine-origin genes. Using insights gleaned from our study of the differential transmissibility of oseltamivir-sensitive and -resistant seasonal A/H1N1 viruses, we propose to explore whether current oseltamivir-resistant swine-origin A/H1N1 isolates are less transmissible than sensitive strains, and, if so, how these viruses might evolve (like seasonal A/H1N1 viruses) to overcome any fitness deficiency that oseltamivir-resistance might confer. In 1973, E.D. Kilbourne wrote that the student of influenza is constantly looking back over his shoulder and asking 'what happened?' in the hope that understanding of past events will alert him to the catastrophes of the future [73]; in 2009, we find ourselves amidst the first influenza pandemic in forty years. It is our intention that, in understanding the recent evolution of oseltamivir-resistant seasonal A/H1N1 viruses, these experiments will not only be immediately applicable to the current A/H1N1 swine-origin influenza pandemic, but also might yield data that could inform our approach to antiviral prophylaxis and treatment in future pandemics. Project Narrative: In 2007, less than 1% of seasonal influenza A/H1N1 viruses were resistant to the oral anti-influenza drug oseltamivir (Tamiflu(r)); by 2009, nearly 100% had acquired a genetic mutation that rendered them drug resistant. We hypothesize that these viruses evolved in such a way that they became more transmissible among humans, and that it was more efficient transmission that allowed them to become so prevalent so quickly. The research we propose seeks to understand the genetic changes that occurred in seasonal A/H1N1 viruses to improve mammalian transmission, and in so doing, to assess the likelihood that oseltamivir-resistant swine-origin A/H1N1 viruses might evolve along a similar path to prevalence.